1. Field of the Invention
The present invention relates to measuring an inductor current in a four-switch buck-boost power converter. In particular, the present invention relates to measuring inductor current using an RC circuit referenced to a virtual ground.
2. Discussion of the Related Art
Four-switch buck-boost power converters are used in many different applications. Such a power converter regulates an output voltage which may be higher than, equal to or lower than the input voltage. A typical four-switch buck-boost power converter has a single inductor and operates synchronously to provide high efficiency over a wide range of load currents. In a power converter, to provide over-current protection, discontinuous-mode operation or current loop regulation, inductor current-sensing is normally required. However, in a four-switch buck-boost power converter, current sensing is made difficult by the high common-mode noise that is often present on both terminals of the inductor.
FIG. 1 shows a first current-sensing technique using sensing resistors in power converter 100. Such a technique is used, for example, in the LM5118 and LM25118 circuits available from Texas Instruments, Inc., Dallas, Tex. As shown in FIG. 1, power converter circuit 100 includes inductor 101, diode 104, sensing resistor 105 and switches 102 and 103. Sensing resistor 105, which is connected in series with diode 104 to one terminal of inductor 101, senses the current in inductor 101 when (and only when) diode 104 is conducting. However, such a configuration cannot sense the peak current in inductor 101.
FIG. 2 shows another inductor current-sensing technique in four-switch power converter 200. Four-switch power converter 200 includes inductor 201, switches 202-205, output capacitor 206 and sensing resistor 207. Sensing resistor 207 senses a valley inductor current in “buck” mode (i.e., when switch 205 is maintained in a constant “on” state) and senses a peak inductor current in “boost” mode (i.e., switch 202 is maintained in a constant “on” state). This current-sensing technique is used in the LTC3780, LTC3789, LT3791, LT8705 circuits available from Linear Technology Corporation, Milpitas, Calif.
The technique of FIGS. 1 and 2 has two drawbacks. First, both sensing resistor 105 of FIG. 1 and sensing resistor 207 of FIG. 2 sense only a portion of their respective inductor currents, as each sensing resistor relies on a switch configuration that allows a current flowing in the respective inductor to flow through the sensing resistor. Second, sensing resistor 105 of FIG. 1 and sensing resistor 207 of FIG. 2 both dissipate power, which may lead to thermal issues in the respective circuits. At the same time, using high-power, precision sensing resistors increases system cost and circuit footprint.
Another current-sensing method, referred to as the “DCR inductor current-sensing scheme” has been widely used in buck or boost converters. FIG. 3 shows one example of the DCR current sensing scheme in a four-switch buck-boost converter 300. As shown in FIG. 3, four-switch buck-boost converter 300 includes switches 305-308, inductor 303 and output capacitor 309. The equivalent DC resistance RDCR of inductor 303 is represented by DCR resistor 304 in FIG. 3. The current in inductor 303 is sensed by providing series-connected sensing resistor 301 and sensing capacitor 302 in parallel to inductor 303 (and equivalent DCR resistor 304). The DCR inductor current-sensing scheme attempts to match the time constant of inductor current iL, given by the ratio of inductance L of inductor 303 to its equivalent DC resistance RDCR (i.e., L/RDCR), by the product of resistance Rs of sensing resistor 301 and capacitance Cs of sensing capacitor 302. Under this scheme the sensed voltage Vsense across sensing capacitor 302 is proportional to the product of inductor current iL and DC resistance RDCR (i.e., Vsense=iL*RDCR). However, as explained in the article “10 MHz Current Mode 4 Switch Buck Boost Converter (4SBBC) for Polar Modulation,” by Park et al., published in the Proceedings of the 23rd Annual Applied Power Electronics Conference, pp-1977-83, the rail-to-rain common mode voltage range and the high common mode noise in the sensed voltage, due to switching in the converter output switches, make the current-sensing circuit complicated and very difficult to implement.